collector/netdev_linux_test.go: Add compatibility tests
collector/netdev_linux_test.go: Add compatibility tests

To prepare for the introduction of new metric names, add tests for the legacy
metric names and values. This will make it easier to ensure that the code that
converts the new metrics to the old ones (for compatibility) behaves correctly.

Signed-off-by: Benoît Knecht <997e838f901d7e35be0bf9323f1af26102b0853d@protonmail.ch>

prometheus/node_exporter,prometheus/node_exporter,prometheus/node_exporter